The present invention relates to a condenser microphone unit using an electret.
FIG. 1 depicts a conventional front electret condenser microphone unit. On the front side of a cylindrical capsule 11 of aluminum a front face plate 11a is integrally formed, a in which a second hole 12 is formed in face plate 11a and a cloth 13 is attached on the front side of the face plate 11a. A diaphragm support metal ring 14 is provided on the peripheral portion on the inner side of the face plate 11a in opposition thereto, and electrically connected thereto. On the other surface of the metal ring 14, opposite to the surface thereof with the face plate 11a, an electret diaphragm 15 is stuck. The electret diaphragm 15 comprises a polymer film, for example, a rather thick FEP (Fluoro Ethylene Propylene) film of 12.5 .mu.m, on one side of which a metal is vapor deposited. The polymer film which is polarized is supported at its periphery by the metal ring 14.
A back electrode 17 is disposed to oppose the diaphragm 15, in close vicinity thereto, via a ring-shaped spacer 16, and the back electrode 17 is held on the front end of a cylindrical back electrode holding member 18. In a back chamber 19 comprised of an interior of the back electrode holding member 18, an impedance conversion IC device 21 is disposed. An input terminal 22 of the IC device 21 is connected to the back electrode 17. An output terminal 23 and a common terminal (not illustrated) are protruded from the rear side of the capsule 11, and connected to wiring on a wiring board 24 that closes the rear side of the capsule 11. On the rear side of the wiring board 24, a rear end portion of the capsule 11 is bent so that each inner part is pressed against the face plate 11a for fixing the entire assembly.
FIG. 2 depicts a conventional back electret type condenser microphone unit. Although the diaphragm itself of the front electret type of FIG. 1 comprises an electret, the back electret type of FIG. 2 is composed of an electret polymer film 26 attached closely to the upper side of the back electrode 17. More explicitly, an electret polymer film, for example, a FEP film is melted to adhere on the upper side of the back electrode 17, and polarized to form an electret. Other details of the composition are the same as those in FIG. 1.
With conventional electret condenser microphone units shown in FIGS. 1 and 2, the back electrode 17 is required in addition to the other various parts. Therefore, it is difficult to assemble the unit automatically, resulting in a limited degree of cost reduction. Because an electret film is used as a diaphragm, there is a limit to making it thinner and therefore, the sensitivity cannot be made higher in excess of a limit.